1. Field of the Invention
The present invention relates to improvements in a DC to AC power inverter. More specifically, the present invention relates to improvements in a load demand sensing circuit of the inverter, a quiescent operating current and feedback circuit for the inverter, and an automatic power factor correction circuit of the inverter.
2. Description of the Prior Art
Heretofore DC to AC inverters were turned on after a load was applied to the inverter, and it was necessary to sense a load applied to the inverter before turning on the inverter. This was typically done by sensing a load using a DC bias on the AC circuit while the AC output of the inverter was turned off and isolated. By sensing a DC current, one could determine if a load had been placed on the inverter. In this way, once a predetermined DC current was sensed, the inverter was turned on, and AC power was delivered to the load. At the same time, a relay was activated to isolate the DC bias so as not to harm the DC current sensing circuit. After the inverter was turned on to supply AC power to the load an AC current sensing circuit was actuated to sense the AC power applied and to keep the AC power output of the inverter turned on.
Once the AC current had fallen below a predetermined value, the AC output power of the inverter was turned off and the DC bias and current sensing circuit was reapplied to the output lines of the inverter.
When using a DC bias and a DC current sensing circuit coupled to the output lines of the inverter to detect the presence of a load, any basically alternating current device such as a wall clock or doorbell transformer appeared as a large load to the DC current sensing circuit because such devices appeared as a short circuit to the DC bias. Accordingly, the inverter was turned on to supply AC current and the AC sensing circuit was sensitive enough to stay on after the DC current sensing circuit caused the inverter to be turned on. Additionally, the AC current sensing circuit was very sensitive so as to detect minute AC current loads which in the case of an old electrical wiring installation, could cause the inverter to stay on even if the actual load is removed. This was due to leakage current in the old electrical wiring. Moreover there were times when long lines to a power tool had sufficient leakage current so as to indicate to the AC current sensing circuit a load which in actuality did not exist.
As will be described in greater detail hereinafter, the inverter of the present invention provides a self-detecting, load demand circuit which cyclically energizes the inverter while at the same time sensing AC current draw from the inverter. If AC current draw is sensed, the load demand circuit is constructed to keep the inverter energizing logic in an on mode to keep the inverter energized.
Heretofore DC to AC power inverters of the class B, C, D or E type, namely those which utilize an SCR, required relatively high input currents at no load. A no load current draw is necessary to establish a capacitor commutation charge in the capacitor circuitry associated with the SCR s. Typically resistive loading is provided to establish the required current draw. Of course, the current drained off the battery is dissipated in the resistors, thereby reducing the batteries useful capability.
As will be described in greater detail hereinafter, the inverter of the present invention provides a feedback circuit through the SCRs establishing satisfactory capacitory communication charge. The current is then, via the primary winding and a feedback winding, stepped to a higher voltage and rectified and returned to the battery. In this way, the current drawn for satisfactory SCR operation is fed back to the battery to significantly decrease battery drain and increase the efficiency of the inverter in a standby or no load mode.
While previous power factor correction circuits have worked well, particularly with smaller capacity inverters, of say 5 kilowatts, such circuits have not worked well with a larger inverter, say on the order of 12 kilowatts.
Also heretofore disadvantages had been incurred with previous power factor correction circuitry. More specifically, previous power factor correction circuitry was not sensitive to light reactive loads and did not feed enough leading power factor correction for large reactive loads such as could be used with a 12 kilowatt inverter. This was due to the fact that the power factor correction was switched in only at the end of a half cycle. As will be described in greater detail hereinafter, the inverter of the present invention includes automatic power factor correcting circuitry which supplies full time leading power factor correction, which is very sensitive to light reactive loads, and works well with inverters of greater than 5 kilowatts capacity.